1. Field of the Invention
This invention relates in general to the field of light water nuclear reactors and more particularly to control rods for light water nuclear reactors.
2. Description of the Prior Art
In the pressure vessel of a commercial pressurized light water reactor, a reactive region commonly referred to as a nuclear core contains fissile fuel such as uranium-235 or plutonium-239 in which sustained fission reactions occur and generate heat. A reactor coolant, comprising, for example, light water, is used to remove the heat generated by the nuclear reaction. The reactor coolant flows into the pressure vessel, through the nuclear core, out of the pressure vessel, through steam generators and then back into the pressure vessel. Another cooling medium which most often is also water, is arranged in heat transfer relationship with the reactor coolant within the steam generator. The secondary cooling medium is converted into steam in the steam generator and is thereafter used to produce electricity by conventional large steam turbine-electrical generator combinations.
In the above-described type of nuclear reactor, control rods are used to control the power output and the power distribution within the nuclear core, and to shut down the nuclear reaction. Control rods typically comprise a plurality of elongated rods which are attached to a shaft and are insertable into the core from outside the pressure vessel. A rod drive mechanism positioned atop of the pressure vessel is used with each control rod assembly to move the control rod into and out of the core.
Control rods comprise or contain materials which absorb neutrons produced by the fission process. Accordingly, in the prior art, materials having a high neutron capture cross section such as boron carbide (B.sub.4 C), hafnium or silver-indium-cadmium have been used. The use of such materials has not been without disadvantage. Boron carbide requires special manufacturing techniques, processes and precautions to assure component integrity. Then too, boron carbide depletes more rapidly than other absorber materials, thereby shortening the useful life of the control rods. Large quantities of helium gas are also released with depletion which result in high internal pressures within the control rod cladding. These shortcomings of B.sub.4 C have led to a preference of the use of hafnium and/or silver-indium-cadmium for control rod materials. Both of these materials, however, are expensive. Also, cadmium and indium are scarce and are on the U.S. Government's list of strategically scarce metals, and, since hafnium and silver-indium-cadmium have high neutron capture cross sections, the introduction of such high neutron absorbers into the core can adversely affect the core. Their high neutron capture cross sections severely depress the power output from the fuel assemblies in which they are located. Therefore, otehr fuel assemblies not having control rods associated therewith are required to produce correspolndingly more power. The mismatch in the power output between the "rodded" and "unrodded" fuel assemblies, which may be adjacent to each other, can be quite large and can restrict the overall power output from the nuclear reactor.
Accordingly, objects of the present invention are to provide a control rod cluster assembly for a light water nuclear reactor which flattens the axial and radial power distribution within the core, reduces the need for high priced control rod materials, provides improved reactor control, provides acceptable shutdown margin and renders less severe various postulated accidents. Other objects not specifically mentioned will be obvious to one skilled in the art to which the invention pertains, which objects are intended to be included within the scope of the present invention.